Mach number

From CFD-Wiki

The Mach number, , is the local flow-speed, , divided by the local
speed of sound, :

The Mach number can be used to categorise compressible flows into different Mach number regimes. To get a more physical insight into Mach number, it is useful if it is viewed as being proportional to the ratio of Kinetic energy to the internal energy of the molecules.